Weight loss: Advantage Qsymia. But we already knew that from the clinical trials.

Retaining patients: Advantage Qsymia. Belviq's label recommends doctors stop treatment if a patient hasn't lost 5% of his or her body weight in 12 weeks. Qsymia is more liberal, only suggesting patients lose 3% of their body weight after 12 weeks of treatment. And then doctors have the choice of prescribing a higher dose of Qsymia before discontinuing treatment.

Ease of prescription: Advantage Belviq. Without a Risk Evaluation and Mitigation Strategy, Belviq will be easier to prescribe than Qsymia, which will be dispensed through specially certified pharmacies to help prevent pregnant patients from taking the drug.

Marketing: Advantage Belviq, for now. Arena is partnered with Eisai, which should be able to put more marketing muscle into Belviq than VIVUS can into Qsymia. But that assumes VIVUS launches the drug and doesn't license it or get acquired.

Price: Advantage unknown. Neither company has released a price yet.

Timing: Advantage Qsymia. Maybe. The drug will launch in the fourth quarter. Arena has to wait for the Drug Enforcement Administration to issue its scheduling of the drug since there's potential for abuse. The DEA decision typically takes four to six months, which would put the launch sometime between the mid-fourth quarter and early 2013. A few months' difference will have minimal effect on the long-term sales trajectory no matter which company comes in first.

Names: Advantage Belviq. Qnexa, the former name of Qsymia, might have won this one, but the name was presumably rejected by the FDA because Qnexa was too similar to the trade or generic name of another drug. The agency wants to avoid having a doctor's bad handwriting cause dispensing errors. But Qsymia? Just saying it makes me feel queasy.

Post-approval requirements: Advantage Belviq. Six post-approval requirements trumps the 10 the FDA is requiring VIVUS to perform. Both companies will have to perform cardiovascular outcomes studies, which will be the greatest cost, so perhaps the overall costs won't be that much.

The first four are clearly the most important. The question becomes whether Belviq's cleaner side-effect profile and potentially stronger marketing muscle can overcome Qsymia's better weight loss. Assuming most patients aren't covered by insurance, the cost of the drugs could be the deciding factor.

Of course, this isn't likely to be a winner-take-all situation. Even Merck's (NYS: MRK) Victrelis, which got trounced by Vertex Pharmaceuticals' (NAS: VRTX) Incivek out of the gate, has managed to grab some sales in the hepatitis C space.

The smackdown is just beginning, but it'll be a while before we can declare a winner.